Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as an electric current producing cell, the electrode on the electrochemically higher potential side is referred to as the positive electrode, or the cathode, while the electrode on the electrochemically lower potential side is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. An electric current producing cell or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, an electric current producing cell comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
As the evolution of batteries continues, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting, high energy density, and light weight batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode active materials and alkali metals as anode active materials for high energy primary and secondary batteries.
To achieve high capacity in electric current producing cells or batteries, it is desirable to have a high quantity or loading of electroactive material in the cathode active layer. For example, the volume of the cathode active layer in an AA size battery is typically about 2 cm.sup.3. If the specific capacity of the electroactive material is a very high value, such as 1000 mAh/g, the amount or volumetric density of the electroactive material in the cathode active layer would need to be at least 500 mg/cm.sup.3 in order to have the 1 gram of cathode active material in the AA size battery necessary to provide a capacity of 1000 mAh. If the volumetric density of electroactive material in the cathode active layer can be increased to higher levels, such as greater than 900 mg/cm.sup.3, the capacity of the battery may be proportionately increased to higher levels if the specific capacity of the electroactive material does not decrease significantly when the cathode active layer becomes denser and less porous.
There are a wide variety of electroactive materials that may be utilized in the cathode active layers of electric current producing cells. For example, a number of these are described in copending U.S. patent application Ser. No. 08/859,996, to Mukherjee et al. to the common assignee. These electroactive materials vary widely in their specific densities (g/cm.sup.3) and in their specific capacities (mAh/g) so the desired volumetric densities in mg/cm.sup.3 of the electroactive material in the cathode active layer correspondingly vary over a wide range. Lithium and sulfur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of electric current producing cells because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium may be present as the pure metal, in an alloy, or in an intercalated form, and the sulfur may be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content, preferably above 75 weight percent sulfur. For example, in combination with a lithium anode, elemental sulfur has a specific capacity of 1680 mAh/g, and sulfur-containing polymers with trisulfide and longer polysulfide groups in the polymers have shown specific capacities of more than 1200 mAh/g. These high specific capacities are particularly desirable for applications, such as portable electronic devices and electric vehicles, where low weight of the battery is important.
Herein, the term "sulfur-containing polymers" pertains to polymers comprising sulfur-sulfur bonds forming trisulfide (--S--S--S--) and higher polysulfide linkages. These sulfur-containing polymers comprise, in their oxidized state, a polysulfide moiety, S.sub.m, selected from the group consisting of covalent --S.sub.m -- moieties, ionic --S.sub.m.sup.- moieties , and ionic S.sub.m.sup.2- moieties, wherein m is an integer equal to or greater than 3. For example, sulfur-containing polymers comprising covalent --S.sub.m -- moieties are described in U.S. Pat. Nos. 5,601,947; 5,690,702; 5,529,860; and copending U.S. Patent application Ser. No. 08/602,323, all to Skotheim et al. Sulfur-containing polymers comprising ionic --S.sub.m.sup.- moieties are described in U.S. Pat. No. 4,664,991 to Perichaud et al. Also, for example, sulfur-containing polymers comprising ionic S.sub.m.sup.- moieties are described in the aforementioned U.S. Pat. No. 4,664,991 and in European Patent No. 250,518 B 1 to Genies. Organo-sulfur materials with only disulfide (--S--S--) moieties typically show specific capacities only in the range of 300 to 700 mAh/g and are accordingly much less desirable for those applications requiring high specific capacities.
It is known to those skilled in the art of battery design and fabrication that practical battery cells comprising the electroactive cathode and anode materials also typically contain other non-electroactive materials such as a container, current collectors, separator, and electrolyte, in addition to polymeric binders, electrically conductive additives, and other additives in the electrodes. The electrolyte is typically an aqueous or nonaqueous liquid, gel, or solid material containing dissolved salts or ionic compounds with good ionic conductance, but with poor electronic conductivity. All of these additional non-electroactive components are typically utilized to make the battery perform efficiently, but they also serve to reduce the gravimetric and volumetric energy density of the cell. It is, therefore, desirable to keep the quantities of these non-electroactive materials to a minimum so as to maximize the amount of electroactive material in the battery cell.
To achieve the highest possible volumetric density of the electroactive material in the cathode active layer, it is desirable to maximize the weight percent for electroactive materials in the cathode active layer, for example, 65 to 85 weight percent of electroactive materials of a specific density of about 2 g/cm.sup.3, such as most high energy density sulfur-containing materials have, and to maintain the porosity or air voids in the cathode active layer as low as possible, for example, in the range of 30 to 60 volume percent. Particularly, the porosity of the cathode active layer must be kept low because higher porosities, such as, for example, 70 to 85 volume percent, do not provide enough electroactive material to obtain very high cell capacities. With the electroactive transition metal oxides, this desirable low porosity is often relatively easy to achieve because these oxides typically have electrically conductive properties and are typically microporous so that high levels of added conductive fillers and microporous additives are not required.
However, with electroactive sulfur-containing materials, which typically have much higher specific capacities than the electroactive transition metal oxides, it is difficult to obtain efficient electrochemical utilization of the sulfur-containing materials at high volumetric densities because the sulfur-containing materials are highly electrically non-conducting or insulative and are generally not microporous. For example, U.S. Pat. Nos. 5,523,179; 5,532,077; 5,582,623; 5,686,201; 5,789,108; and 5,814,420; to Chu, describe the problems of overcoming the insulating character of elemental sulfur in composite cathodes and the use of a homogeneous distribution of an electronically conductive material, such as carbon black, and of an ionically conductive material together with the elemental sulfur in the composite cathode to try to overcome these problems. To retain the homogeneous distribution of the particles and ionically conductive material, the cathode coatings in the above-mentioned patents to Chu are described as being dried in such a manner that the electrode components do not significantly redistribute, such as, for example, by evaporation of the volatile liquids from the cathode coatings at room temperature as, for example, also described in the examples of the aforementioned patents to Chu. The preferred amounts of ionically conductive material, such as polyethylene oxide with an ionic salt, in the cathode active layer in the above-mentioned patents to Chu are 15 to 75 percent by weight in order to achieve at least a 5% electrochemical utilization of the elemental sulfur. All of the examples in the above- mentioned patents to Chu contain either 45% or 50% by weight of elemental sulfur in the cathode active layer. Although the weight per cm.sup.2 of the cathode active layer is reported in some of these examples, it is not possible to calculate the volumetric density of elemental sulfur in the cathode active layer because the thickness of the cathode active layer is not reported for any of the examples. Thus, although the volumetric density of elemental sulfur in the cathode active layer in the examples in the aforementioned patents to Chu is not reported, it is likely to be lower than 500 mg/cm.sup.3 due to the relatively low amount of elemental sulfur in the cathode active layer and the relatively large amount of ionically conductive material in the cathode active layer. Because of its lower specific density than elemental sulfur, ionic conductive materials, such as polyethylene oxide with lithium salts, occupy a greater volume percent of the cathode active layer than their weight percent value and tend to increase the thickness of the cathode active layer and thus fuirther lower the volumetric density of the elemental sulfur in the cathode active layer. Similarly, U.S. Pat. No. 4,664,991 to Perichaud et al. describes sulfur-containing polymers comprising ionic --S.sub.m.sup.- moieties with a linear conductive polymer backbone in cathode active layers where a high electrochemical utilization, such as a specific capacity of about 1000 mAh/g, was achieved with less than 50 weight percent of the sulfur-containing polymer in the cathode active layer. The weight per cent of the sulfur-containing polymer was reduced below the 50 percent by weight level by the addition of undisclosed amounts of either a polytetraethylene polymer or a blend of polyethylene oxide and a lithium salt into the composition of the cathode active layer. The '991 patent only reports the weight per cm.sup.2 of the cathode active layer and does not report its thickness so that it is not possible to calculate the volumetric density of the electroactive sulfur-containing polymer in the cathode active layer. Also, the '991 patent does not report the drying conditions for the coating of the cathode active layer.
To overcome the insulative properties of electroactive sulfur-containing materials, large amounts of electrically conductive fillers, such as conductive carbons, are typically added to the cathode active layer. Typically, the electrically conductive fillers are present in the amounts of about 5 to 40% by weight of the cathode active layer. For example, U.S. Pat. No. 3,639,174 to Kegelman describes solid composite cathodes comprising elemental sulfur and a particulate electrical conductor. U.S. Pat. No. 4,303,748 to Armand et al. describes solid composite cathodes containing an ionically conductive polymer electrolyte together with elemental sulfur, transition metal salts, or other cathode active materials for use with lithium or other anode active materials. U.S. Pat. No. 5,460,905 to Skotheim describes the use of p-doped conjugated polymers, together with an effective amount of conductive carbon pigments, for the transport of electrons in sulfur-based cathodes. U.S. Pat. No. 5,529,860 and U.S. patent application Ser. No. 08/602,323, both to Skotheim et al., describe the use of conductive carbons and graphites, conductive polymers, and metal fibers, powders, and flakes with electroactive sulfur-containing materials comprising covalent --S.sub.m.sup.- moieties , wherein m is an integer equal to or greater than 3.
It would be advantageous to significantly increase the volumetric densities of cathode active layers comprising electroactive sulfur-containing materials without sacrificing the high specific capacity of these materials, i.e., without reducing the desired high electrochemical utilization, such as, for example, greater than 50% utilization, during cycling of the cells. Particularly as the thickness of the cathode active layer is increased, it becomes progressively more difficult to achieve the electrical conductivity and the microporosity needed for highly efficient electrochemical utilization of the sulfur-containing materials. For example, it would be very beneficial to the capacity of batteries if the volumetric density of the cathode active layer, when the layer has a desired loading of electroactive sulfur-containing material in mg/cm.sup.2 and the desired high electrochemical utilization, could be increased by a factor of 50% or 100% or even more than 100% from the volumetric densities of 400 mg/cm.sup.3 or less which are typical, for example, of cathode active layers comprising 50% by weight of elemental sulfur, 5 to 40% by weight of conductive fillers, and 5 to 40% by weight of ionicially conductive materials, polymeric binders, and other non-electroactive materials.
One method to increase the volumetric density of the cathode active layer is by compressing or calendering the layer to a reduced thickness. It would be very advantageous to be able to compress or calender the cathode active layer to a 20% or greater reduction in thickness without sacrificing the desired high electrochemical utilization of the electroactive sulfur-containing materials. This is difficult to achieve when the high levels of non-electroactive materials are present in the cathode active layer, particularly when polymeric binders and any ionically conductive materials are present which typically also provide binding properties to the cathode active layer, such that the electrochemical utilization, as expressed in the specific capacity of the electroactive sulfur-containing material in the cell, is typically significantly reduced when the cathode active layer is reduced in thickness by compressing or calendering. On the other hand, significantly reducing the levels of the non-electroactive materials in the cathode active layer, particularly those materials with binding properties, greatly reduces the mechanical integrity and cohesive and adhesive properties of the cathode active layer. Elemental sulfur and sulfur-containing polymers comprising a polysulfide moiety are typically present in the cathode active layer as particles that do not have binding or film-forming properties so that polymeric binders, including ionically conductive materials, are typically added to provide the cohesion and adhesion properties to the cathode active layer needed for mechanical integrity during the manufacturing and use of the batteries comprising the cathode active layer.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells comprising elemental sulfur or other electroactive sulfur-containing materials, there remains a need for improved solid composite cathodes comprising a cathode active layer which has a combination of high electrochemical utilization at a high volumetric density of the electroactive sulfur-containing material, while retaining or improving the desirable properties of electrical conductivity, mechanical strength, the ability to be compressed without any significant mechanical damage, cohesive strength, and adhesion to the adjacent layers in the solid composite cathodes utilizing electroactive sulfur-containing materials, such as, for example, elemental sulfur and sulfur-containing polymers comprising a polysulfide moiety.